feat: add node-express-mongo docs

[sarthaksarthak9]

Pull Request Template

Description

This pr serves as a guide for developers to set up a sample CRUD application using Node.js, Express.js, and MongoDB, and leverage Keploy for effortless API testing and Jest integration for unit testing and code coverage.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue).
• [ ] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [X] This change requires a documentation update.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Additional Context (Please include any Screenshots/gifs if relevant)

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Checklist:

• [X] My code follows the style guidelines of this project.
• [X] I have performed a self-review of my own code.
• [X] I have made corresponding changes to the documentation.
• [ ] I have added corresponding tests.
• [ ] Any dependent changes have been merged and published in downstream modules.